Info

*Center-to-side flow. tSide-tocenter flow.

*Center-to-side flow. tSide-tocenter flow.

Comment. This check indicates that at turned-down conditions, downcomer backup exceeds the clearance under the downcomer by more than 2 in. Therefore, no seal loss problem is expected.

6.5.10 Concluding comments on design philosophy

The example reflects the prime difficulty often encountered by tray designers: inconsistent predictions from different correlations. The three entrainment flood correlations used gave predictions that widely differed; the differences were up to 50 to 60 percent. Another inconsistency was in the weep-dump prediction. These inconsistencies stem from the empiricism associated with prediction methods. Our understanding of tray hydrodynamics has still a very long way to go before it can provide us with models that reliably predict tray performance from first principles.

In the meantime, we (as designers) have to make the best of the em pirical world we live in. The safest approach is to base the design on the most conservative prediction. At times, this is also the best approach. The decision of whether to go for a conservative prediction depends on consequences of failure, cost of conservatism, and confidence in prediction. For instance, downcomer area in this example was conservatively sized. Figure 6.8 shows that in the high-pressure region, downcomers are likely to bottleneck column throughput. Designing these too tight can lead to a premature bottleneck. The consequences of failure are too harsh, and our understanding of downcomer hydraulics is too incomplete to justify a nonconservative approach. Further, the costs of oversizing the downcomers are relatively low. Therefore, the benefits outweigh the costs, and a conservative design was justified.

In two other cases, nonconservative decisions were made, namely in weeping and entrainment flooding prediction. In the case of weeping, the correlation inconsistencies could have been resolved at relatively low cost by going to valve trays. Going to valve trays here would have been quite a reasonable design decision. The author, however, felt confident with the weep-rate predictions (both of the methods used were tested at high pressure, Sec. 6.2.12). Further, the consequences of failure can be lived with at least temporarily, and a later retray with valve trays is not too expensive. On the other hand, when it came to entrainment flooding prediction, a lot was at stake. A conservative prediction here would have grossly oversized the column, and therefore, would have had a very high price tag. Considering the limitations of the correlations used (Sec. 6.2.6) there was a good basis to believe that there was no need to go for the most conservative correlation. Noticing that capacity at high pressure is normally limited by downcomer rather than entrainment flooding (Fig. 6.8) gives one more reason for selecting a nonconservative approach.

The example reflects another philosophical aspect of the design: how to optimize sections of the column that have excess capacity. In this case, it was the top section. As a first step, the example minimized the excess capacity by cutting back on tray spacing. This shortens column height and directly saves costs. As a second step, the tray design was adjusted to give as much margin as possible from potential bottlenecks, e.g., by generously sizing downcomers and by increasing downcomer clearance. During the life of a column, feeds, loads, and capacity requirements change. A good tray design will think ahead and attempt to eliminate any future bottlenecks without incurring a substantial cost penalty. Debottlenecking at a future date is far more expensive. In practice, optimization of column sections that have excess capacity is one aspect of tray design that is often neglected. It is the author's hope that this example will help increase awareness of this aspect.

6.5.11 Tray design summary

Tower diameter, ft Tray spacing, in

Type of tray Hole diameter, in Tray thickness Number of passes

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